Scanning electron microscope



Oct. 21, 1969 HIROKAZU KIMURA ETAL.

SGANNING ELECTRON MIcRoscoPE Filed June 21, 1966 3 Sheets-Sheet l Oct21. 1969 HiRoKAzu KIMURA ETAL. 3,474,245

SCANNING ELECTHON MICROSCOPE Filed June 21, 1966 3 Sheets-Sheet b w) 3Am 0,7 mmv Km W K m -WH w X /0 4 C 0 7. 6. 5 4. 5 2 0 0 0 0. 0 0 0. 0 mbubmmQ INVENTOR n n ORNEY United States Patent O 3,474,245 SCANNINGELECTRON MICROSCOPE Hirokazu Kimura, Koganei-shi, Hifumi Tamura,Hachiojishi, Mi'chiyoshi Maki, Kodaira-shi, andHisayuki Higuch,Kokubunji-shi, Japan, assignors to Hitachi, Ltd., Tokyo, Japan, acorporation of Japan -Filed June 21, 1966, Ser. No. 559,296 Claimspriority, application Japan, June 23, 1965,

Int. c1. Holi 37/26 Us. c1. o-49,5 9 claims ABSTRACT 0F THE DISCLOSUREThis invention relates to improvements in the structure of scanningelectron microscopes and more particularly to a scanning electronmicroscope' provided with novel means for detecting secondary electrons.

As is commonly known, a scanning electron microscope operates in such away that an electron beam having a fine'spot, by being finely focused,is used to scan the surface of a specimen in its longitudinal andlateral directions and reflected electrons or secondary electronsemitted from points of the specimen bombarded by the primary electronbeam are detected in the form of signals, which signals are then`supplied to a cathode-ray tube, which is scanned in synchronizedrelation with the above scanning by the primary electron beam, so as tomodulatethe brightness vof the cathode-ray tube and to thereby observethe surface image of the specimen on the fluorescent screen. In -thescanning electron microscope, its resolution is dependent upon thediameter of focused electron beam. It is therefore desirable to minimizethe diameter of electron beam spot in order to obtain a high resolutionimage. To this end, it is necessary that the focusing electron lens hasa short focal length and this means that the distance between thefocusing electron lens and a specimen for microscopic examination mustbe as short as possible. In ordinary scanning electron microscopes,however, a detector for refiected electrons or secondary electrons hasbeen located between Ithe focusing electron lens and an observedspecimen, with the result that it has generally been difficult to use anelectron lens of short focal length and such difficulty has beenespecially marked when an electron lens of shor-t focal length isintended for use with a detector for secondary electrons.

With such prior difficulty in mind, it is the primary object of thepresent invention to provide a unique arrangement by which secondaryelectrons and reflected electrons emitted from a specimen by beingbombarded by the primary electron beam can be induced to any desiredposition outside of the path of the primary electron beam.

Another object of the present invention is to provide the capability` ofusing a focusing electron lens of short focal length having littlespherical aberration and astigmatism, whose capability is derivable fromlthe attainment of the first-described object.

i A further object of the present invention is to derive 3,474,245Patented Oct. 21, 1969 ICC a high resolution image by focusing theprimary electron beam to a diameter of the order of several tens toseveral hundreds of angstroms by the use of the above-described focusingelectron lens with short focal length.

In order to attain the objects as described above, the scanning electronmicroscope according to the present invention is composed of means foremitting a primary electron beam, focusing electron lens means forfocusing Said primary electron beam or thereby illuminating a specimen,means for directing said primary electron beam to the specimen for thescanning thereof, means for establishiug a first uniform magnetic fieldwhich is disposed in close proximity to the magnetic field establishedby said focusing lens means and extends linearly along the path of saidprimary electron beam, means for establishing a second uniform magneticfield in such a manner that it forms a substantially continuousextension of said first magnetic field and extends outwardly away fromthe path of said primary electron beam, means for detecting thosesecondary electrons and/or reflected electrons which are derivedoutwardly after passing through said first and second magnetic fields,means operative in response to a detected electronic signal to modulatethe brightness of the scanning electron beam in a cathoderay tube, andmeans for deflecting said scanning electron beam in synchronous relationwith the scanning by said primary electron beam.

In another form of the present invention, the scanning electronmicroscope is composed of means for emitting a primary electron beam,focusing lens means for focusing said primary electron beam for therebyilluminating a Specimen, means for directing said primary electron beamto the specimen for the scanning thereof, means for establishing a rstuniform magnetic field which is disposed in close proximity to themagnetic field established by said focusing lens means and extendslinearly along the path of said primary electron beam, means forestablishing a static field so that the secondary electrons and/orreflected electrons having passed through said first magnetic field canthereby be deflected and derived outwardly of the path of said primaryelectron beam, means for detecting the derived secondary electronsand/or reliected electrons, means operative in response to a detectedelectric signal to modulate the brightness of the scanning electron beamin a cathode-ray tube, and means for defiecting said scanning electronbeam in synchronous relation with the scanning by said primary electronbeam.

The above and other objects, advantages and features of the presentinvention will become obvious from the following description withreference to the accompanying drawings, in which:

FIG. l is a diagrammatic view showing` the structure of a prior form ofscanning electron microscope;

FIG. 2 is a schematic perspective View showing the structure of a priorsystem used for the detection of secondary electrons;

FIG. 3 is a schematic sectional view showing the structure of part of anembodiment according to the invention;

FIG. 4 is an enlarged view of part of the embodiment shown in FIG. 3 forthe sake of explanation of the operating principle of the microscopeaccording to the invention;

FIG. 5 is a schematic sectional view of part of another embodimentaccording to the invention;

FIG. 6 is a graphic illustration of test results obtained with theembodiment shown in FIG. 5; and

FIG. 7 is a schematic sectional View showing the structure of a furtherembodiment according to the invention.

yBefore giving any detailed description of the present invention,structure of a prior form of scanning electron microscope will first bedescribed with reference to FIG. 1 so that the improvements effected bythe present invention can more clearly be understood.

A scanning electron microscope of prior form is disclosed for example inthe Journal of Scientific Instruments, 1960, vol. 37, pages 246248 andin the Journal of Scientific Instruments, 1965, vol. 42, pages 81-85 andgenerally has a structure as shown in FIG. 1. This scanning electronmicroscope includes electron beam emission means consisting of a cathode1 in the form of a hair-pin shaped tungsten filament, a grid 2 generallycalled the Wehnelt cylinder, and an anode 3. A voltage of 20 to 30kilovolts is applied -across the anode 3 and the cathode 1, and anelectron beam 4 emitted from the heated tungsten filament is focused ona specimen 7 to be observed by means of two condenser lenses 5 and 6.The specimen 7 is placed on a specimen stage 8 and the specimen positionis adjustable in both horizontal and axial directions. The electron beam4 is deflected by two pairs of defiecting plates 9 so as to scan thespecimen surface longitudinally and laterally thereof. When the specimensurface is bombarded by the primary electron beam 4, a secondaryelectron beam 10 is emitted from the bombarded point and the amount ofthe secondary electrons is variable depending on the material of thatparticular bombarded point and the incident angle of the primaryelectron beam 4. This secondary electron beam 10 generally has an energyof less than 50 electronvolts. A mesh 11 having a central aperture forthe passage therethrough of the incident beam is disposed opposite thespecimen 7 and is kept at a negative potential of from several toseveral tens of volts so that the secondary electron beam 10 isdeflected in a manner as shown and only those secondary electrons havinga certain constant energy are passed through a slit 12. These secondaryelectrons are then accelerated to hit against a scintillator 13, and asignal whose amplitude is proportional to the number of electrons havingpassed through the slit 12 is detected by a photomultiplier 14. Thissignal is then supplied through an amplifier 15 to a grid 17 of acathode-ray tube 16 for effecting modulation of the brightness of thetube 16. Meanwhile, the output of the scanning power source 18 issimultaneously supplied to the defiecting plates 9 of the scanningelectron microscope column, and to defiecting plates 19 of thecathoderay tube 16. By this simultaneous supply of scanning power, asecondary electron image of the specimen 7 can be observed on thefluorescent screen of the cathoderay tube 16.

In addition to the above-described secondary electron detection systemin which the energy of the emitter secondary electrons is analyzed andthose secondary electrons having a specific energy are solely detected,a simpler form of secondary electron detection system has previouslybeen used and generally has a structure as shown in FIG. 2. Thissecondary electron detection system includes a mesh 23 which has acentral aperture 24 for passage therethrough of an incident electronbeam 22 and is electrically insulated from the ground by insulators 25.A secondary electron beam 26 emitted from an observed specimen 21 istrapped by the mesh 23 to be derived in the form of a signal current 27.In this case, reflected electrons having high energies pass through thismesh 23 and do not appear as a signal.

It will be seen that, in both of prior manners of secondary electrondetection, their secondary electron detection system occupies aconsiderable space with the result that the second focusing lens 6 musthave a large focal length of the order of several ten millimeters. Thislarge focal length is undesirable in view of the fact that the influenceof spherical aberration and astigmatism of the electron lens becomesquite great and that the minimum diameter of the electron beam will onlybe of the order of 0.1 micron.

Improvements effected by the present invention over the prior art willnow be described in detail with reference to FIGS. 3 to 7.V Referringfirst to FIG. 3, a second focusing lens includes an upper magnetic pole31 and a lower magnetic pole 32 disposed opposite to each other with aspacer 33 of non-magnetic material interposed therebetween. A coil 34establishes a magnetic field of several thousand gauss across this lensgap. Further, a solenoid coil 35 is disposed adjacent the above magneticfield and consists of a straight portion 35 (FIG. 4) of suitable lengthand a curved portion 35" which is gradually bent towards a detector 36disposed outside of the path of a primary electron beam 38 so thatsecondary electrons (or reflected electrons) emitted from a specimen 39can be guided into the detector 36. Other parts of the embodiment shownin FIG. 3 have a structure generally similar to that shown in FIG. 1.

An aperture 37 is provided at a suitable portion of the solenoid coil 35to permit passage therethrough of the incident primary electron beam 38.The primary electron beam 38 is focused onto the specimen 39 by themagnetic field established by the focusing lens and its point beingbombarded on the specimen surface is successively longitudinally andlaterally moved by defiecting plates 44 for the scanning of the specimen39. Secondary electrons or refiected electrons 40 emitted from the pointbombarded by the primary electron beam are worked upon by the magneticfield established by the focusing lens and then by the succeedingmagnetic field established by the solenoid coil 35, with the result thatthese electrons 40 are confined substantially in the vicinity of thecentral axis of the solenoid coil 35 to move along a spiral path, `asshown by a dotted line, into the detector 36. A movable aperture 41 isdisposed in the vicinity of the central axis of the focusing electronlens and is adapted to be controlled from outside of vacuum. Anaccelerating electrode 42 for the acceleration of secondary electrons orrefiected electrons is disposed in close proximity to the specimensurface and is led outside of the vacuum to have a positive potential offrom several to several tens of volts. An electrical insulator 43insulates the accelerating electrode 42 from the focusing electron lens.Another accelerating electrode 45 is disposed at the outlet of thesolenoid coil 35 to accelerate the secondary electrons or refiectedelectrons leaving the solenoid coil 35. The

r specimen 39 is placed on a specimen stage 46 and its position isvertically and horizontally adjustable from the exterior.

The principle of operation of the embodiment shown in FIG. 3 will bedescribed with reference to FIG. 4. As Will be apparent from FIG. 4,secondary electrons or refiected electrons 40 having an initial velocityv are emitted from the specimen 39 when the primary electron beam 38hits against the specimen 39. A magnetic field 47 is established in thevicinity of the specimen 39 by the straight portion 35 of the solenoidcoil 35. For ease of explanation, the velocity v of the secondaryelectron or reected electron 40 is divided into two velocity components,that is, a velocity component vy in the same direction as that of themagnetic field having a field strength H1 and a velocity component vx ina direction at right angles with respect to the velocity component vy.It is then known that the secondary electron or reflected electron 40makes a gyrating motion since the velocity component vX is perpendicularto the direction of the magnetic field while the velocity component vyis entirely free from the effect of the magnetic field. The radius r1 ofgyration of the secondary electron or refiected electron 40 is given bythe following formula:

mUx GHI along a spiral path 48 while being confined within a cylindricalarea substantially coaxial with the ,central axislof the solenoid coil35 and having a diameter of from `severalto several tens of microns.

Suppose then that the curved portion 35" of the solenoid coil 35 isdisposed at an angle 0 with respect to the straight portion 35' of thesolenoid coil 35. Then, a velocity u of the secondary electron orreflected electron 40 coming into the solenoid coil portion 35'.' at anangle a with respect to the central axis thereof can .likewise bedivided into two velocity components, that is, a velocity component uyin the axial direction of the coil portion 35" and avelocity componentux in a direction at right angles with respect to the component uy.Suppose the solenoid coil portion 35" establishes a magnetic fieldhaving a field strength H2, then the radius r2 of gyration of thesecondary electron or reliected electron is likewise given by a formulamax SH2 In this manner, the secondary electrons or reflectedelectronsadvance in the axial direction of the solenoid coil 35 whiledepicting a spiral trajectory, and it is therefore possible to guide thesecondary electrons or reflected electrons to vany desired place outsidethe path of the primaryelectron beam. In the above embodiment, theprimary electron beam 38 for bombarding the specimen 39 maybe admittedthrough the gap between the coil portions 35 and 35'. instead of throughthe aperture 37 shown in FIG. 3.

Another embodiment according to theinvention is shown .in FIG. 5 inwhich three coils 49, 50 and 51 of beam 38 is focused on a specimen 39by this electron lens and is deflected by the deecting plates 44disposed outside the solenoid coil 58 so that the specimen 39 can belongitudinally and laterally scanned by the primary electron beam 38 ina manner as described previously. A secondary electron or reflectedelectron beam 40 derived from each point struck by the primary electronbeam 38 flies while depicting a spiral trajectory about the central axisof the solenoid coil 58 in a manner as described previously. Thesecondary electron or reflected electron beam 40 is then accelerated byan accelerating electrode 59 disposed in the vicinity of the outlet ofthe solenoid coil 58 and kept at a positive potential of several elec- Vtron volts and its electron trajectory is deflected by an same. shapeare employed in deriving secondary electrons or reflected electrons froma specimen 39 bombarded by a primary electron beam 38. Each of thesecoils 49, 50 and 51 consists of 300 turns of wire wound to an insidediameter of 10 mm., an outside diameter of 18 mm. and a heightof l2 mm.The primary electron beam 38 strikes the specimen 39, which is a pieceof silicon, through the ,coils 49 and :50, and a secondary electron orreflected electron beam is` derived from the specimen 39. The secondaryelectrons or reflected electrons pass 'through the coils 49, 50 and 51while gyrating substantially about lthe central axis of these coils forthe reason as described previously and finally reach a detectory or aFaradays cup 46. The Faradays cup 46 is grounded' through an ammeter 52and a power supply 53 at several volts of .positive value.'FIG. 6 showsthe relation between the exciting currentsupplied to the abovethreefcoilsin series andthe detected current when the acceleratingvoltage and the electron currentv of the primary electron beam are-20kilovoltsand 4 109 ampere, respectively. From. FIG. 6 it will be seenthat the detected current increase monotonically with the increase inthe coil current and finally reaches a value of 0.6\ 10-9 ampere at acoil current, of 300 milliamperes in spite of the fact that a darkcurrent of 0.05 10-9 ampere can only be obtained at zero coil current.The result shown in FIG. 6 apparently proves the fact that the secondaryelectron current-passes through the three successive coils to reach theFaradays cup 46. In this connection, there maybe the fear thatv theprimary electron beam is deopposite electrode at a negative potential offrom several to several tens of electron volts, with the result thatonly those electrons having a certain specific energy pass through aslit 61 to reach a detector 62.

The just-described embodiment is advantageous over the embodiment shownin FIG. 3 in that the incident primary electron beam does not pass anaperture provided on the side face of the solenoid' coil but passesthrough a magnetic field of axial symmetry and hence the electron beamis not subjected to any deflection or distortion by a non-uniformmagnetic field.

It is to be understood that the detectors 36, 46 and 62 appearing in theembodiments of the invention may be detectors of secondary electrons orreflected electrons or detectors of both of these electrons. It isapparent that the present invention is also applicable to scanningelectron microscopes of the type employing only one focusing lens, andis applicable to electron beam devices of the type analogous to thedevice of the present invention in the entirely same manner. Forexample, marked effects substantially similar to those obtained by theinvention can be expected when the present invention is applied to anX-ray microanalyser of the kind disclosed in The Elion Reports, vol. l,No. 3, May 1961, or an electron beam machining apparatus of the kindappearing in FIGS. 5 and 6 on page 295 of the Proceedings of the ThirdSymposium on Electron Beam Processes, March 23 and 24, 1961.

From the foregoing description it will be appreciated that, according tothe invention, the specimen can be positioned in close proximity to thefocusing lens in the scanning electron microscope since secondaryelectrons or reflected electrons derived from the observed specimen canbe guided to any desired place outside of the path-of the primaryelectron beam for the detection thereof. This specific feature permitsthe use of a focusing lens of short focal length and thus minimizes thespherical aberration and astigmatism inherent in the lens. As a resultof such unique feature, the diameter of the electron beam spot can bereduced to such a small value of ilected by the magnetic fieldsestablished by the solenoids, but such deflection measured. on thespecimen was only about 0.5 mm. and was thus negligible as a matter ofpractical use. 1 A- j In a further embodiment of the invention shown inFIG. 7, an electron lens is formed by an upper magnetic pole piece 54and a lower magnetic pole piece 55 With a spacer 56 of non-magneticmaterial interposed therebetween. A coil 57 on the electron lens excitesthe same. A solenoid coil 58 is interposed in the magnetic path in apartly overlapped relation with the magnetic liield established acrossthe electron lens gap. A primary electron the order of from severalvtens to several hundreds of angstroms and a high resolution image canthereby be obtained.

What is claimed is:

1.' In an electron microscope including electron beam generating meansfor generating and directing a beam of primary electrons along a beampath toward a point at which a specimen may be supported and at leastone focusing means positioned closely adjacent said point between saidbeam generating means and the specimen for focusing said electron beamto a spot on the specimen, the improvement comprising detector meansdisplaced from said beam path and 1ocated at a distance from saidfocusing means between said focusing means and said beam generatingmeans for detecting secondary and reflected electrons emitted from thespecimen as a result of bombardment thereof by said beam of primaryelectrons, and field generating means for guiding said secondary andreflected electrons from the specimen to said detector means, at least apart of the path between the specimen and said detector means beingconcident with said beam path of the primary electrons on the side ofsaid focusing means opposite the specimen.

2. The combination defined in claim 1 wherein said field generatingmeans includes first means for generating a magnetic field along a pathbetween the specimen and said detector means.

3. The combination defined in claim 1 wherein said first means isprovided as a continuous coil extending from said focusing means to saiddetector means and being provided with an aperture therein for passageof said beam of primary electrons to the specimen.

4. The combination defined in claim 3 wherein said field generatingmeans further includes an accelerating electrode positioned at each endof said coil.

5. The combination defined in claim 1 wherein said first means includesa plurality of coils having their axes aligned with the path between thespecimen and said detector means.

6. The combination defined in claim 1 wherein said first means isprovided as a cylindrical coil coaxial with said beam path of theprimary electrons, and further including means for defiecting saidsecondary and reflected electrons from the end of said coil opposite theend adjacent said focusing means to said Idetector means.

7. The combination defined in claim 1 wherein said first means includesmeans for establishing a first uniform magnetic field which is disposedin close proximity to the magnetic field generated by said focusingmeans and extends linearly along said beam path and means forestablishing a second uniform magnetic field forming a substantiallycontinuous extension of said first magnetic field and extendingoutwardly away from said beam path of primary electrons.

8. In a scanning electron microscope:

beam generating means for emitting a primary electron beam,

support means for supporting a specimen,

at least one focusing electron lens of the magnetic type positionedbetween said beam generating means and said support means for focusingthe primary electron beam onto the specimen,

first ldeflecting means positioned between said beam generating meansand said focusing means for defiecting the primary electron beam to scanthe specimen,

detector means for detecting electrons which are emitted from theportion of the specimen struck by the primary electron beam,

a cathode-ray tube having a scanning electron beam for displaying animage of the specimen, means for modulating the intensity of thescanning electron beam in the cathode-ray tube in response to the outputsignal of said detecting means, and

second defiecting means for deilecting the scanning electron beam in thecathode-ray tube in synchronous relation to the scanning operation ofthe primary electron beam,

the improvement comprising:

said support means being disposed in the vicinity of the central axis ofthe focusing electron lens with the specimen positioned within thehousing of the focusing electron lens,

first solenoid coil disposed in close proximity to the specimen and onthe primary electron beam incident side of the focusing electron lensfor developing a first uniform magnetic field extending linearly alongthe path of the primary electron beam to guide the electrons emitted bythe vspecimen alongspiral paths in the vicinity of the central axis ofthe first solenoid coil, and

a second solenoid coil disposed in close proximity to the first solenoidcoil for developing a second uniform magnetic field contiguous to thefirst uniform magnetic field to guide the electrons emitted by thespecimen to the detector means.

9. In a scanning electron microscope:

beam generating means for emitting a primary electron beam,

support means for supporting a specimen,

at least one focusing electron lens 'of the magnetic type positionedbetween said beam generating means and said support means for focusingthe primary electron beam on the specimen,

first defiecting means positioned between said beam generating means andsaid focusing means for de- -flecting the primary electron beam to scanthe speci-l men,

detector means for detecting electrons which are emitted from theportion of the specimen struck by the primary electron beam,

a cathode-ray tube having a scanning electron beam for displaying animage of the specimen,

means for modulating the intensity of the scanning electron beam in thecathode-ray tube in response to the output signal of said detectormeans, and

second deflecting means for deffecting the scanning electron beam in thecathode-ray tube in synchronous relation with the scanning operation ofthe primary electron beam,

the improvement comprising:

the support means being disposed in the vicinity of the central axis ofthe focusing electron lens with the specimen positioned within thehousing of the focusing electron lens,

a solenoid coil disposed in close proximity to the specimen and on theprimary electron beam incident side of the focusing electron lens fordeveloping a uniform magnetic field .extending linearly along the pathof the primary electron beam to guide the electrons emitted by thespecimen along spiral paths in the vicinity of the central axis of thefirst solenoid coil,

a first electrode disposed in close proximity to the solenoid coil foraccelerating the electrons passed through the solenoid coil in adirection opposite to the primary electron beam, and

a second electrode disposed opposite to the first electrode fordeflecting the electrons accelerated by the first electrode to directthe accelerated electrons to the ,detector means,

the first and second electrodes having apertures through which theprimary electron beam passes.

References Cited UNITED STATES PATENTS 11/1956 Warmoltz. 12/1965 Shapiroet al.

OTHER REFERENCES Journal of Scientific Instruments, 1960, vol. 37, pp.246-248.

